A. Field of the Invention
The present invention relates generally to systems for monitoring environmental contaminants and, more particularly, to systems for measuring fugitive emissions from process equipment.
B. Description of the Related Art
Industrial plants that handle volatile organic compounds (VOCs) typically experience unwanted emissions of those compounds into the atmosphere from point sources such as smokestacks and non-point sources such as valves, pumps, and fittings installed in pipes and vessels containing the VOCs. Emissions from non-point sources, referred to as xe2x80x9cfugitivexe2x80x9d emissions, typically occur due to leakage of the VOCs from joints and seals. Fugitive emissions from control valves may occur as leakage through the packing between the valve stem and body/bonnet of the valve. Valves employed in demanding service conditions involving frequent movement of the valve stem and large temperature fluctuations typically suffer accelerated deterioration of the valve stem packing, resulting in greater fugitive emissions than valves in less demanding service.
While improvements in valve stem packing materials and designs have reduced fugitive emissions and lengthened the life of valve packing, the monitoring of fugitive emissions has become important as a means to identify and reduce fugitive emissions and comply with new more stringent regulation of emissions. The Environmental Protection Agency (EPA) has promulgated regulations specifying the maximum permitted leakage of certain hazardous air pollutants from control valves, and requiring periodic surveys of emissions from control valves.
Current methods of monitoring fugitive emissions involve manual procedures using a portable organic vapor analyzer. This manual method is time consuming and expensive to perform, and can also yield inaccurate results due to ineffective collection of the fugitive emissions from the equipment being monitored. If measurements are made on a valve exposed to wind, emissions from the valve may be dissipated before the vapor analyzer can properly measure the concentration of the emissions. Also, if the analyzer is not carefully moved around the valve to capture all the emissions from the valve, an inaccurate measurement will result. Manual measurement methods also require plant personnel to dedicate a significant amount of time to making the measurements, distracting from their other duties.
Automated monitoring and detection of fugitive emissions can yield significant advantages over existing manual methods. The EPA regulations require surveys of fugitive emissions at periodic intervals. The length of the survey interval may be monthly, quarterly, semi-annual, or annual; the required surveys becoming less frequent if the facility operator can document fewer than a certain percentage of control valves with excessive leakage. Thus, achieving a low percentage of leaking valves reduces the number of surveys required per year. In a large industrial facility where the total number of survey points can range from 50,000 to 200,000 points, this can result in large cost savings. By installing automated fugitive emission sensing systems onto valves subject to the most demanding service conditions and thus most likely to develop leaks, compliance with the EPA regulations can be more readily achieved for the entire facility. This results in longer intervals between surveys for all of the valves, significantly reducing the time and expense of taking measurements manually from the valves without automated sensing systems.
Early detection of fugitive emissions from leaking valves also enables repairs to be made on a more timely basis, reducing the quantity of hazardous material emitted and reducing the cost of lost material. Accurate sensing of fugitive emissions provides an early warning system which can alert the facility operator to a potential valve seal failure and enable preventive measures to be taken before excessive leakage occurs.
However, employing an automated fugitive emission sensing system in an industrial environment requires designing a sample retrieval system which can efficiently collect fugitive emissions emanating from a piece of equipment and transport the emissions to gas sensors. The sample retrieval system must be capable of delivering a sample stream at a known flow rate in order to permit the gas sensors to make accurate and consistent measurements of the concentration of fugitive emissions.
Furthermnore, employing gas sensors in an industrial environment requires designing sensors that perform satisfactorily in the presence of high relative humidity (up to 85%) through a broad temperature range (from xe2x88x9240xc2x0 C. to +85xc2x0 C.). The sensors must be able to discriminate between the emissions of interest and other environmental contaminants, while retaining sufficient sensitivity to detect low concentrations of the fugitive emissions. Provision also must be made to enable periodic calibration of the gas sensors. The output signals from the fugitive emission sensing system must be suitable for input into plant monitoring and control systems typically found in process plants. This will permit simple and inexpensive integration of the sensing system into existing plant process control systems.
The fugitive emission sensing system must be inexpensive to manufacture, and use a power source that is readily available in a typical process plant, in order to keep installation costs to a minimum. The system must be suitable for use in hazardous areas subject to a risk of explosion, requiring electrical equipment to be of intrinsically safe or explosion-proof design. It also must be able to operate in harsh environments, including areas subject to spray washing, high humidity, high and low temperatures, and vibration. The system also must be simple and reliable, in order to keep maintenance costs to a minimum.
Accordingly, it is an object of the present invention to provide an apparatus and method that addresses the concerns set forth above.
According to one aspect of the present invention, a system for collecting data relating to emissions from an emissions source comprises an accumulator adapted to receive emissions from the emissions source, a sensor in flow communication with an outlet of the accumulator for generating a signal indicative of a physical property of the emissions, and a sensor interface circuit receiving the signal and generating data relating to the emissions from the emissions source. In a particular embodiment, the accumulator comprises a collecting tube, and in another embodiment, the accumulator comprises a bonnet capsule.
In accordance with another aspect of the invention, the system includes an ejector in flow communication with the outlet of the accumulator. The ejector draws the emissions from the accumulator to expose the sensor to the emissions. The ejector may be connected to a source of pressurized fluid so that the pressurized fluid flows through the ejector thereby creating a pressure drop to draw the emissions from the accumulator into the ejector.
In accordance with another aspect of the invention, the system includes a sensor calibrator in flow communication with the at least one sensor for storing a calibrant and exposing the at least one sensor to the calibrant.
In accordance with another aspect of the invention, the system provides that data generated by the sensor interface circuit is derived by measuring the frequency of said signal generated by the sensor.
In accordance with another aspect of the invention, the system includes a microcontroller adapted to receive the data from the sensor interface circuit, and a memory connected to the microcontroller for storing data from the sensor interface circuit where the data is derived from the at least one sensor""s response to the calibrant.
In accordance with another aspect of the invention, a system for reducing emissions from an emissions source comprises an accumulator adapted to receive emissions from the emissions source, a sensor in flow communication with an outlet of the accumulator for generating a signal indicative of a physical property of the emissions, a sensor interface circuit receiving the signal for generating data relating to the emissions from the emissions source, and a microcontroller receiving the data for generating control signals for reducing emissions from the emissions source.
In accordance with another aspect of the invention, a method for collecting data relating to emissions from an emissions source comprises collecting at least a portion of the emissions, exposing at least one sensor to the emissions to generate a signal indicative of a physical property of the emissions, and processing the signal generated by the at least one sensor to generate data relating to the emissions from the emissions source.
In accordance with another aspect of the invention, a method for reducing emissions from an emissions source comprises situating an accumulator adjacent the emissions source to receive the emissions, providing at least one sensor in flow communication with the accumulator, exposing the at least one sensor to the emissions to generate a signal indicative of a physical property of the emissions, and processing the signal generated by the at least one sensor to generate control signals for controlling plant conditions to reduce the emissions from the emissions source.